Reusable substrate for thin film separation

ABSTRACT

A donor substrate (10) for forming multiple thin films of material (12). In one embodiment, a first thin film of material is separated or cleaved from a donor substrate by introducing energetic particles (22) through a surface of a donor substrate (10) to a selected depth (20) underneath the surface, where the particles have a relatively high concentration to define donor substrate material (12) above the selected depth. Energy is provided to a selected region of the substrate to cleave a thin film of material from the donor substrate. Particles are introduced again into the donor substrate underneath a fresh, or cleaved, surface of the donor substrate. A second thin film of material is then cleaved from the donor substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from the provisional patent applicationentitled A CONTROLLED CLEAVAGE PROCESS AND RESULTING DEVICE, filed May12, 1997 and assigned application Ser. No. 60/046,276, the disclosure ofwhich is hereby incorporated in its entirety for all purposes. Thisapplication is being filed on the same date as related application Ser.No. 09/026,032 entitled "A PRESSURIZED MICROBUBBLE THIN FILM SEPARATIONPROCESS USING A REUSABLE SUBSTRATE" and application Ser. No. 09/026,113entitled "A CONTROLLED CLEAVAGE THIN FILM SEPARATION PROCESS USING AREUSABLE SUBSTRATE".

BACKGROUND OF THE INVENTION

The present invention relates to the manufacture of substrates. Moreparticularly, the invention provides a reusable donor substrate forseparating a thin film of material from. The thin film can be used inthe fabrication of a silicon-oninsulator substrate for semiconductorintegrated circuits, for example. But it will be recognized that theinvention has a wider range of applicability; it can also be applied toother substrates for multi-layered integrated circuit devices,three-dimensional packaging of integrated semiconductor devices,photonic devices, piezoelectronic devices, microelectromechanicalsystems ("MEMS"), sensors, actuators, solar cells, flat panel displays(e.g., LCD, AMLCD), biological and biomedical devices, and the like.

Wafers for electronic device fabrication are often cut from an ingot, orboule, of material with an abrasive saw. The wafer often serves as botha mechanical substrate and a semiconductor material to form electronicdevices in or on. One of the most common examples of this is cuttingsilicon wafers from a silicon ingot. The wafers are typically polishedto a very fine surface finish after removing the mechanical damage leftby the abrasive saw. In some processes, devices are fabricated directlyin or on the silicon wafer. In other processes, a layer of semiconductormaterial is grown, for example by epitaxy, on the wafer. An epitaxiallayer may provide lower impurity concentrations, or be of a differentsemiconductor type than the wafer. The devices are formed in what isknown as the "active" layer, which is typically only a micron or sothick.

Sawing wafers from an ingot has several disadvantages. First, asignificant amount of material may be lost due to the width, or kerf, ofthe saw blade. Second, the wafers must be cut thick enough to survive atypical circuit fabrication process. As the wafers get larger andlarger, the required thickness to maintain sufficient strength to becompatible with given wafer handling methods increases. Third, thepolishing process to remove the saw marks takes longer and removes yetmore precious material than would be required if an alternative methodexisted.

The desire to conserve material lost to the sawing and polishingoperations increases as the value of an ingot increases. Single-crystalsilicon ingots are now being produced with diameters of twelve inches.Each wafer cut and polished from these ingots can cost over a thousanddollars. Ingots of other materials are also being produced. Some ofthese materials may be difficult to produce as a single crystal, or mayrequire very rare and expensive starting materials, or consume asignificant amount of energy to produce. Using such valuable material toprovide simple mechanical support for the thin active layer is veryundesirable, as is losing material to the sawing and polishingoperations.

Several materials are processed by cleaving, rather than sawing.Examples include scribing and breaking a piece of glass, or cleaving adiamond with a chisel and mallet. A crack propagates through thematerial at the desired location to separate one portion of materialfrom another. Cleaving is especially attractive to separate materialsthat are difficult to saw, for example, very hard materials. Althoughthe cleaving techniques described above are satisfactory, for the mostpart, as applied to cutting diamonds or household glass, they havesevere limitations in the fabrication of semiconductor substrates. Forinstance, the above techniques are often "rough" and cannot be used withgreat precision in fabrication of the thin layers desired for devicefabrication, or the like.

From the above, it is seen that a technique for separating a thin filmof material from a substrate which is cost effective and efficient isoften desirable.

SUMMARY OF THE INVENTION

According to the present invention, a technique for removing thin filmsof material from a reusable substrate is provided. This techniqueseparates thin films of material from a donor substrate by implantingparticles, such as hydrogen ions, into the donor substrate, and thenseparating the thin film of material above the layer of implantedparticles. A second implant and separation process is then performed toremove multiple films from a single substrate.

In a specific embodiment, the present invention provides a process forforming a film of material from a donor substrate, which is reusable,using a controlled cleaving process. That process includes a step ofintroducing energetic particles (e.g., charged or neutral molecules,atoms, or electrons having sufficient kinetic energy) through a surfaceof a donor substrate to a selected depth underneath the surface, wherethe particles are at a relatively high concentration to define athickness of donor substrate material (e.g., thin film of detachablematerial) above the selected depth. To cleave the donor substratematerial, the method provides energy to a selected region of the donorsubstrate to initiate a controlled cleaving action in the donorsubstrate, whereupon the cleaving action is made using a propagatingcleave front(s) to free the donor material from a remaining portion ofthe donor substrate. The remaining portion of the donor substrate isreused in another cleaving process, if desired.

In another embodiment, a layer of microbubbles is formed at a selecteddepth in the substrate. The substrate is globally heated and pressure inthe bubbles eventually shatters the substrate material generally in theplane of the microbubbles.

The present invention separates several thins films of material from asingle, reusable donor substrate. The thin films can be used forfabrication of, for example, a silicon-on-insulator orsilicon-on-silicon wafer. A planarizing layer of silicon oxide may beformed on the donor substrate after each cleaving step to facilitatebonding the donor wafer to a transfer wafer, or stiffener. Accordingly,the present invention provides a reusable substrate, thereby savingcosts and reduces the amount of scrap material.

In most of the embodiments, a cleave is initiated by subjecting thematerial with sufficient energy to fracture the material in one region,causing a cleave front, without uncontrolled shattering or cracking. Thecleave front formation energy (E_(c)) must often be made lower than thebulk material fracture energy (E_(mat)) at each region to avoidshattering or cracking the material. The directional energy impulsevector in diamond cutting or the scribe line in glass cutting are, forexample, the means in which the cleave energy is reduced to allow thecontrolled creation and propagation of a cleave front. The cleave frontis in itself a higher stress region and once created, its propagationrequires a lower energy to further cleave the material from this initialregion of fracture. The energy required to propagate the cleave front iscalled the cleave front propagation energy (E_(p)). The relationship canbe expressed as:

    E.sub.c =E.sub.p +[cleave front stress energy]

A controlled cleaving process is realized by reducing E_(p) along afavored direction(s) above all others and limiting the available energyto be below the E_(p) of other undesired directions. In any cleaveprocess, a better cleave surface finish occurs when the cleave processoccurs through only one expanding cleave front, although multiple cleavefronts do work.

This technique uses a relatively low temperature during the controlledcleaving process of the thin film to reduce temperature excursions ofthe separated film, donor substrate, or multi-material films accordingto other embodiments. This lower temperature approach allows for morematerial and process latitude such as, for example, cleaving and bondingof materials having substantially different thermal expansioncoefficients. In other embodiments, the present invention limits energyor stress in the substrate to a value below a cleave initiation energy,which generally removes a possibility of creating random cleaveinitiation sites or fronts. This reduces cleave damage (e.g., pits,crystalline defects, breakage, cracks, steps, voids, excessiveroughness) often caused in pre-existing techniques. Moreover, thepresent invention reduces damage caused by higher than necessary stressor pressure effects and nucleation sites caused by the energeticparticles as compared to pre-existing techniques.

The present invention achieves these benefits and others in the contextof known process technology. However, a further understanding of thenature and advantages of the present invention may be realized byreference to the latter portions of the specification and attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 are simplified diagrams illustrating a controlled cleavingtechnique according to an embodiment of the present invention;

FIGS. 11-15 are simplified cross-sectional view diagrams illustrating amethod of forming a silicon-on-insulator substrate according to thepresent invention; and

FIGS. 16-19 are simplified diagrams illustrating a method of separatinga second layer of material from a substrate.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention provides a technique for removing a thin film ofmaterial from a substrate while preventing a possibility of damage tothe thin material film and/or a remaining portion of the substrate. Thethin film of material is attached to or can be attached to a targetsubstrate to form, for example, a silicon-on-insulator wafer. The thinfilm of material can also be used for a variety of other applications.The invention will be better understood by reference to the Figs. andthe descriptions below.

1. Controlled Cleaving Techniques

FIG. 1 is a simplified cross-sectional view diagram of a substrate 10according to the present invention. The diagram is merely anillustration and should not limit the scope of the claims herein. Asmerely an example, substrate 10 is a silicon wafer which includes amaterial region 12 to be removed, which is a thin relatively uniformfilm derived from the substrate material. The silicon wafer 10 includesa top surface 14, a bottom surface 16, and a thickness 18. Substrate 10also has a first side (side 1) and a second side (side 2) (which arealso referenced below in the Figs.). Material region 12 also includes athickness 20, within the thickness 18 of the silicon wafer. The presentinvention provides a novel technique for removing the material region 12using the following sequence of steps.

Selected energetic particles implant 22 through the top surface 14 ofthe silicon wafer to a selected depth 24, which defmes the thickness 20of the material region 12, termed the thin film of material. A varietyof techniques can be used to implant the energetic particles into thesilicon wafer. These techniques include ion implantation using, forexample, beam line ion implantation equipment manufactured fromcompanies such as Applied Materials, Eaton Corporation, Varian, andothers. Alternatively, implantation occurs using a plasma immersion ionimplantation ("PIII") technique or an ion shower technique. Examples ofplasma immersion implantation techniques are described in "RecentApplications of Plasma Immersion Ion Implantation," Paul K. Chu, ChungChan, and Nathan W. Cheung, SEMICONDUCTOR INTERNATIONAL, pp. 165-172,June 1996, and "Plasma Immersion Ion Implantation--A Fledgling Techniquefor Semiconductor Processing,", P. K. Chu, S. Qin, C. Chan, N. W.Cheung, and L. A. Larson, MATERIALS SCIENCE AND ENGINEERING REPORTS: AREVIEW JOURNAL, pp. 207-280 Volume R17, Nos. 6-7, (Nov. 30, 1996), whichare both hereby incorporated by reference for all purposes. Of course,techniques used depend upon the application.

Depending upon the application, smaller mass particles are generallyselected to reduce a possibility of damage to the material region 12.That is, smaller mass particles easily travel through the substratematerial to the selected depth without substantially damaging thematerial region that the particles traverse through. For example, thesmaller mass particles (or energetic particles) can be almost anycharged (e.g., positive or negative) and/or neutral atoms or molecules,or electrons, or the like. In a specific embodiment, the particles canbe neutral and/or charged particles including ions such as ions ofhydrogen and its isotopes, rare gas ions such as helium and itsisotopes, and neon. The particles can also be derived from compoundssuch as gases, e.g., hydrogen gas, water vapor, methane, and hydrogencompounds, and other light atomic mass particles. Alternatively, theparticles can be any combination of the above particles, and/or ionsand/or molecular species and/or atomic species. The particles generallyhave sufficient kinetic energy to penetrate through the surface to theselected depth underneath the surface.

Using hydrogen as the implanted species into the silicon wafer as anexample, the implantation process is performed using a specific set ofconditions. Implantation dose ranges from about 10¹⁵ to about 10¹⁸atoms/cm², and preferably the dose is greater than about 10¹⁶ atoms/cm².Implantation energy ranges from about 1 KeV to about 1 MeV, and isgenerally about 50 KeV. Implantation temperature ranges from about -200to about 600° C., and is preferably less than about 400° C. to prevent apossibility of a substantial quantity of hydrogen ions from diffusingout of the implanted silicon wafer and annealing the implanted damageand stress. The hydrogen ions can be selectively introduced into thesilicon wafer to the selected depth at an accuracy of about +/-0.03 to+/-0.05 microns. Of course, the type of ion used and process conditionsdepend upon the application.

Effectively, the implanted particles add stress or reduce fractureenergy along a plane parallel to the top surface of the substrate at theselected depth. The energies depend, in part, upon the implantationspecies and conditions. These particles reduce a fracture energy levelof the substrate at the selected depth. This allows for a controlledcleave along the implanted plane at the selected depth. Implantation canoccur under conditions such that the energy state of substrate at allinternal locations is insufficient to initiate a non-reversible fracture(i.e., separation or cleaving) in the substrate material. It should benoted, however, that implantation does generally cause a certain amountof defects (e.g., micro-detects) in the substrate that can be repairedby subsequent heat treatment, e.g., thermal annealing or rapid thermalannealing.

FIG. 2 is a simplified energy diagram 200 along a cross-section of theimplanted substrate 10 according to the present invention. The diagramis merely an illustration and should not limit the scope of the claimsherein. The simplified diagram includes a vertical axis 201 thatrepresents an energy level (E) (or additional energy) to cause a cleavein the substrate. A horizontal axis 203 represents a depth or distancefrom the bottom of the wafer to the top of the wafer. After implantingparticles into the wafer, the substrate has an average cleave energyrepresented as E 205, which is the amount of energy needed to cleave thewafer along various cross-sectional regions along the wafer depth. Thecleave energy (E_(t)) is equal to the bulk material fracture energy(E_(mat)) in non-implanted regions. At the selected depth 20, energy(E_(cz)) 207 is lower since the implanted particles essentially break orweaken bonds in the crystalline structure (or increase stress caused bya presence of particles also contributing to lower energy (E_(cz)) 207of the substrate) to lower the amount of energy needed to cleave thesubstrate at the selected depth. This takes advantage of the lowerenergy (or increased stress) at the selected depth to cleave the thinfilm in a controlled manner.

FIG. 3 is a simplified cross-sectional view of an implanted substrate 10using selective positioning of cleave energy according to the presentinvention. This diagram is merely an illustration, and should not limitthe scope of the claims herein. The implanted wafer undergoes a step ofselective energy placement or positioning or targeting which provides acontrolled cleaving action of the material region 12 at the selecteddepth. The impulse or impulses are provided using energy sources.Examples of sources include, among others, a chemical source, amechanical source, an electrical source, and a thermal sink or source.The chemical source can include a variety such as particles, fluids,gases, or liquids. These sources can also include chemical reaction toincrease stress in the material region. The chemical source isintroduced as flood, time-varying, spatially varying, or continuous. Inother embodiments, a mechanical source is derived from rotational,translational, compressional, expansional, or ultrasonic energies. Themechanical source can be introduced as flood, time-varying, spatiallyvarying, or continuous. In further embodiments, the electrical source isselected from an applied voltage or an applied electromagnetic field,which is introduced as flood, time-varying, spatially varying, orcontinuous. In still further embodiments, the thermal source or sink isselected from radiation, convection, or conduction. This thermal sourcecan be selected from, among others, a photon beam, a fluid jet, a liquidjet, a gas jet, an electromagnetic field, an electron beam, athermoelectric heating, a furnace, and the like. The thermal sink can beselected from a fluid jet, a liquid jet, a gas jet, a cryogenic fluid, asuper-cooled liquid, a thermoelectric cooling means, an electro/magneticfield, and others. Similar to the previous embodiments, the thermalsource is applied as flood, time-varying, spatially varying, orcontinuous. Still further, any of the above embodiments can be combinedor even separated, depending upon the application. Of course, the typeof source used depends upon the application.

In a specific embodiment, the present invention provides acontrolledpropagating cleave. The controlled-propagating cleave usesmultiple successive impulses to initiate and perhaps propagate acleaving process 700, as illustrated by FIG. 4. This diagram is merelyan illustration, and should not limit the scope of the claims herein. Asshown, the impulse is directed at an edge of the substrate, whichpropagates a cleave front toward the center of the substrate to removethe material layer from the substrate. In this embodiment, a sourceapplies multiple pulses (i.e., pulse 1, 2, and 3) successively to thesubstrate. Pulse 1 701 is directed to an edge 703 of the substrate toinitiate the cleave action. Pulse 2 705 is also directed at the edge 707on one side of pulse 1 to expand the cleave front. Pulse 3 709 isdirected to an opposite edge 711 of pulse 1 along the expanding cleavefront to further remove the material layer from the substrate. Thecombination of these impulses or pulses provides a controlled cleavingaction 713 of the material layer from the substrate.

FIG. 5 is a simplified illustration of selected energies 800 from thepulses in the preceding embodiment for the controlled-propagatingcleave. This diagram is merely an illustration, and should not limit thescope of the claims herein. As shown, the pulse 1 has an energy levelwhich exceeds average cleaving energy (E), which is the necessary energyfor initiating the cleaving action. Pulses 2 and 3 are made using lowerenergy levels along the cleave front to maintain or sustain the cleavingaction. In a specific embodiment, the pulse is a laser pulse where animpinging beam heats a selected region of the substrate through a pulseand a thermal pulse gradient causes supplemental stresses which togetherexceed cleave formation or propagation energies, which create a singlecleave front. In preferred embodiments, the impinging beam heats andcauses a thermal pulse gradient simultaneously, which exceed cleaveenergy formation or propagation energies. More preferably, the impingingbeam cools and causes a thermal pulse gradient simultaneously, whichexceed cleave energy formation or propagation energies.

Optionally, a built-in energy state of the substrate or stress can beglobally raised toward the energy level necessary to initiate thecleaving action, but not enough to initiate the cleaving action beforedirecting the multiple successive impulses to the substrate according tothe present invention. The global energy state of the substrate can beraised or lowered using a variety of sources such as chemical,mechanical, thermal (sink or source), or electrical, alone or incombination. The chemical source can include a variety such asparticles, fluids, gases, or liquids. These sources can also includechemical reaction to increase stress in the material region. Thechemical source is introduced as flood, time-varying, spatially varying,or continuous. In other embodiments, a mechanical source is derived fromrotational, translational, compressional, expansional, or ultrasonicenergies. The mechanical source can be introduced as flood,time-varying, spatially varying, or continuous. In further embodiments,the electrical source is selected from an applied voltage or an appliedelectromagnetic field, which is introduced as flood, time-varying,spatially varying, or continuous. In still further embodiments, thethermal source or sink is selected from radiation, convection, orconduction. This thermal source can be selected from, among others, aphoton beam, a fluid jet, a liquid jet, a gas jet, an electro/magneticfield, an electron beam, a thermoelectric heating, and a furnace. Thethermal sink can be selected from a fluid jet, a liquid jet, a gas jet,a cryogenic fluid, a super-cooled liquid, a thermoelectric coolingmeans, an electro/magnetic field, and others. Similar to the previousembodiments, the thermal source is applied as flood, time-varying,spatially varying, or continuous. Still further, any of the aboveembodiments can be combined or even separated, depending upon theapplication. Of course, the type of source used also depends upon theapplication. As noted, the global source increases a level of energy orstress in the material region without initiating a cleaving action inthe material region before providing energy to initiate the controlledcleaving action.

In a specific embodiment, an energy source elevates an energy level ofthe substrate cleave plane above its cleave front propagation energy butis insufficient to cause self-initiation of a cleave front. Inparticular, a thermal energy source or sink in the form of heat or lackof heat (e.g., cooling source) can be applied globally to the substrateto increase the energy state or stress level of the substrate withoutinitiating a cleave front. Alternatively, the energy source can beelectrical, chemical, or mechanical. A directed energy source providesan application of energy to a selected region of the substrate materialto initiate a cleave front which selfpropagates through the implantedregion of the substrate until the thin film of material is removed. Avariety of techniques can be used to initiate the cleave action. Thesetechniques are described by way of the Figs. below.

FIG. 6 is a simplified illustration of an energy state 900 for acontrolled cleaving action using a single controlled source according toan aspect of the present invention. This diagram is merely anillustration, and should not limit the scope of the claims herein. Inthis embodiment, the energy level or state of the substrate is raisedusing a global energy source above the cleave front propagation energystate, but is lower than the energy state necessary to initiate thecleave front. To initiate the cleave front, an energy source such as alaser directs a beam in the form of a pulse at an edge of the substrateto initiate the cleaving action. Alternatively, the energy source can bea cooling fluid (e.g., liquid, gas) that directs a cooling medium in theform of a pulse at an edge of the substrate to initiate the cleavingaction. The global energy source maintains the cleaving action whichgenerally requires a lower energy level than the initiation energy.

An alternative aspect of the invention is illustrated by FIGS. 7 and 8.FIG. 7 is a simplified illustration of an implanted substrate 1000undergoing rotational forces 1001, 1003. This diagram is merely anillustration, and should not limit the scope of the claims herein. Asshown, the substrate includes a top surface 1005, a bottom surface 1007,and an implanted region 1009 at a selected depth. An energy sourceincreases a global energy level of the substrate using a light beam orheat source to a level above the cleave front propagation energy state,but lower than the energy state necessary to initiate the cleave front.The substrate undergoes a rotational force turning clockwise 1001 on topsurface and a rotational force turning counter-clockwise 1003 on thebottom surface which creates stress at the implanted region 1009 toinitiate a cleave front. Alternatively, the top surface undergoes acounter-clockwise rotational force and the bottom surface undergoes aclockwise rotational force. Of course, the direction of the forcegenerally does not matter in this embodiment.

FIG. 8 is a simplified diagram of an energy state 1100 for thecontrolled cleaving action using the rotational force according to thepresent invention. This diagram is merely an illustration, and shouldnot limit the scope of the claims herein. As previously noted, theenergy level or state of the substrate is raised using a global energysource (e.g., thermal, beam) above the cleave front propagation energystate, but is lower than the energy state necessary to initiate thecleave front. To initiate the cleave front, a mechanical energy meanssuch as rotational force applied to the implanted region initiates thecleave front. In particular, rotational force applied to the implantedregion of the substrates creates zero stress at the center of thesubstrate and greatest at the periphery, essentially being proportionalto the radius. In this example, the central initiating pulse causes aradially expanding cleave front to cleave the substrate.

The removed material region provides a thin film of silicon material forprocessing. The silicon material possesses limited surface roughness anddesired planarity characteristics for use in a silicon-on-insulatorsubstrate. In certain embodiments, the surface roughness of the detachedfilm has features that are less than about 60 nm, or less than about 40nm, or less than about 20 nm. Accordingly, the present inventionprovides thin silicon films which can be smoother and more uniform thanpre-existing techniques.

In a preferred embodiment, the present invention is practiced attemperatures that are lower than those used by pre-existing techniques.In particular, the present invention does not require increasing theentire substrate temperature to initiate and sustain the cleaving actionas pre-existing techniques. In some embodiments for silicon wafers andhydrogen implants, substrate temperature does not exceed about 400° C.during the cleaving process. Alternatively, substrate temperature doesnot exceed about 350° C. during the cleaving process. Alternatively,substrate temperature is kept substantially below implantingtemperatures via a thermal sink, e.g., cooling fluid, cryogenic fluid.Accordingly, the present invention reduces a possibility of unnecessarydamage from an excessive release of energy from random cleave fronts,which generally improves surface quality of a detached film(s) and/orthe substrate(s). Accordingly, the present invention provides resultingfilms on substrates at higher overall yields and quality.

The above embodiments are described in terms of cleaving a thin film ofmaterial from a substrate. The substrate, however, can be disposed on aworkpiece such as a stiffener or the like before the controlled cleavingprocess. The workpiece joins to a top surface or implanted surface ofthe substrate to provide structural support to the thin film of materialduring controlled cleaving processes. The workpiece can be joined to thesubstrate using a variety of bonding or joining techniques, e.g.,electro-statics, adhesives, interatomic. Some of these bondingtechniques are described herein. The workpiece can be made of adielectric material (e.g., quartz, glass, sapphire, silicon nitride,silicon dioxide), a conductive material (silicon, silicon carbide,polysilicon, group III/V materials, metal), and plastics (e.g.,polyimide-based materials). Of course, the type of workpiece used willdepend upon the application.

Alternatively, the substrate having the film to be detached can betemporarily disposed on a transfer substrate, such as a stiffener or thelike, before the controlled cleaving process. The transfer substratejoins to a top surface or implanted surface of the substrate having thefilm to provide structural support to the thin film of material duringcontrolled cleaving processes. The transfer substrate can be temporarilyjoined to the substrate having the film using a variety of bonding orjoining techniques, e.g., electrostatics, adhesives, interatomic. Someof these bonding techniques are described herein. The transfer substratecan be made of a dielectric material (e.g., quartz, glass, sapphire,silicon nitride, silicon dioxide), a conductive material (silicon,silicon carbide, polysilicon, group III/V materials, metal), andplastics (e.g., polyimide-based materials). Of course, the type oftransfer substrate used will depend upon the application. Additionally,the transfer substrate can be used to remove the thin film of materialfrom the cleaved substrate after the controlled cleaving process.

2. Another Cleaving Technique

An example of an alternative technique which may form multiple cleavefronts in a random manner is described in U.S. Pat. No. 5,374,564, whichis in the name of Michel Bruel ("Bruel"), and assigned to Commissariat Al'Energie Atomique in France. Bruel generally describes a technique forcleaving an implanted wafer by global thermal treatment (i.e., thermallytreating the entire plane of the implant) using thermally activateddiffusion. The technique described in Bruel implants gas-forming ionsinto a silicon wafer to form a layer of microbubbles, and attaches astiffener to the surface of the wafer. A global thermal treatment of thesubstrate generally causes a pressure effect in the layer ofmicrobubbles that initiates multiple cleave fronts which propagateindependently to separate a thin film of material, which is joined tothe stiffener, to be separated from the substrate. This process resultsin a thin film of material with a rough surface finish on the surface ofthe cleaved material. It is believed that the rough surface results fromthe energy level for maintaining the cleave exceeding the amountrequired, and that the stiffener is important in maintaining theintegrity of the film through the separation process.

3. Silicon-On-Insulator Process

A process for fabricating a silicon-on-insulator substrate according tothe present invention may be briefly outlined as follows:

(1) Provide a donor silicon wafer (which may be coated with a dielectricmaterial);

(2) Introduce particles into the silicon wafer to a selected depth todefme a thickness of silicon film;

(3) Provide a target substrate material (which may be coated with adielectric material);

(4) Bond the donor silicon wafer to the target substrate material byjoining the implanted face to the target substrate material;

(5) Increase global stress (or energy) of implanted region at selecteddepth without initiating a cleaving action (optional);

(6) Provide stress (or energy) to a selected region of the bondedsubstrates to initiate a controlled cleaving action at the selecteddepth;

(7) Provide additional energy to the bonded substrates to sustain thecontrolled cleaving action to free the thickness of silicon film fromthe silicon wafer (optional);

(8) Complete bonding of donor silicon wafer to the target substrate; and

(9) Polish a surface of the thickness of silicon film.

The above sequence of steps provides a step of initiating a controlledcleaving action using an energy applied to a selected region(s) of amulti-layered substrate structure to form a cleave front(s) according tothe present invention. This initiation step begins a cleaving process ina controlled manner by limiting the amount of energy applied to thesubstrate. Further propagation of the cleaving action can occur byproviding additional energy to selected regions of the substrate tosustain the cleaving action, or using the energy from the initiationstep to provide for further propagation of the cleaving action. Thissequence of steps is merely an example and should not limit the scope ofthe claims defined herein. Further details with regard to the abovesequence of steps are described in below in references to the Figs.

FIGS. 9-15 are simplified cross-sectional view diagrams of substratesundergoing a fabrication process for a silicon-on-insulator waferaccording to the present invention. The process begins by providing asemiconductor substrate similar to the silicon wafer 2100, as shown byFIG. 9. Substrate or donor includes a material region 2101 to beremoved, which is a thin relatively uniform film derived from thesubstrate material. The silicon wafer includes a top surface 2103, abottom surface 2105, and a thickness 2107. Material region also includesa thickness (z₀), within the thickness 2107 of the silicon wafer.Optionally, a dielectric layer 2102 (e.g., silicon nitride, siliconoxide, silicon oxynitride) overlies the top surface of the substrate.The present process provides a novel technique for removing the materialregion 2101 using the following sequence of steps for the fabrication ofa silicon-on-insulator wafer.

Selected energetic particles 2109 implant through the top surface of thesilicon wafer to a selected depth, which defines the thickness of thematerial region, termed the thin film of material. As shown, theparticles have a desired concentration 2111 at the selected depth (z₀).A variety of techniques can be used to implant the energetic particlesinto the silicon wafer. These techniques include ion implantation using,for example, beam line ion implantation equipment manufactured fromcompanies such as Applied Materials, Eaton Corporation, Varian, andothers. Alternatively, implantation occurs using a plasma immersion ionimplantation ("PIll") technique. Of course, techniques used depend uponthe application.

Depending upon the application, smaller mass particles are generallyselected to reduce a possibility of damage to the material region. Thatis, smaller mass particles easily travel through the substrate materialto the selected depth without substantially damaging the material regionthat the particles traversed through. For example, the smaller massparticles (or energetic particles) can be almost any charged (e.g.,positive or negative) and/or neutral atoms or molecules, or electrons,or the like. In a specific embodiment, the particles can be neutraland/or charged particles including ions of hydrogen and its isotopes,rare gas ions such as helium and its isotopes, and neon. The particlescan also be derived from compounds such as gases, e.g., hydrogen gas,water vapor, methane, and other hydrogen compounds, and other lightatomic mass particles. Alternatively, the particles can be anycombination of the above particles, and/or ions and/or molecular speciesand/or atomic species.

The process uses a step of joining the implanted silicon wafer to aworkpiece or target wafer, as illustrated in FIG. 10. The workpiece mayalso be a variety of other types of substrates such as those made of adielectric material (e.g., quartz, glass, silicon nitride, silicondioxide), a conductive material (silicon, polysilicon, group III/Vmaterials, metal), and plastics (e.g., polyimide-based materials). Inthe present example, however, the workpiece is a silicon wafer.

In a specific embodiment, the silicon wafers are joined or fusedtogether using a low temperature thermal step. The low temperaturethermal process generally ensures that the implanted particles do notplace excessive stress on the material region, which can produce anuncontrolled cleave action. In one aspect, the low temperature bondingprocess occurs by a self-bonding process. In particular, one wafer isstripped to remove oxidation therefrom (or one wafer is not oxidized). Acleaning solution treats the surface of the wafer to form O--H bonds onthe wafer surface. An example of a solution used to clean the wafer is amixture of H₂ O₂ --H₂ SO₄. A dryer dries the wafer surfaces to removeany residual liquids or particles from the wafer surfaces. Self-bondingoccurs by placing a face of the cleaned wafer against the face of anoxidized wafer.

Alternatively, a self-bonding process occurs by activating one of thewafer surfaces to be bonded by plasma cleaning. In particular, plasmacleaning activates the wafer surface using a plasma derived from gasessuch as argon, ammonia, neon, water vapor, and oxygen. The activatedwafer surface 2203 is placed against a face of the other wafer, whichhas a coat of oxidation 2205 thereon. The wafers are in a sandwichedstructure having exposed wafer faces. A selected amount of pressure isplaced on each exposed face of the wafers to self-bond one wafer to theother.

Alternatively, an adhesive disposed on the wafer surfaces is used tobond one wafer onto the other. The adhesive includes an epoxy,polyimide-type materials, and the like. Spin-on-glass layers can be usedto bond one wafer surface onto the face of another. These spin-on-glass("SOG") materials include, among others, siloxanes or silicates, whichare often mixed with alcohol-based solvents or the like. SOG can be adesirable material because of the low temperatures (e.g., 150 to 250°C.) often needed to cure the SOG after it is applied to surfaces of thewafers.

Alternatively, a variety of other low temperature techniques can be usedto join the donor wafer to the target wafer. For instance, anelectro-static bonding technique can be used to join the two waferstogether. In particular, one or both wafer surface(s) is charged toattract to the other wafer surface. Additionally, the donor wafer can befused to the target wafer using a variety of commonly known techniques.Of course, the technique used depends upon the application.

After bonding the wafers into a sandwiched structure 2300, as shown inFIG. 11, the method includes a controlled cleaving action to remove thesubstrate material to provide a thin film of substrate material 2101overlying an insulator 2305 the target silicon wafer 2201. Thecontrolled-cleaving occurs by way of selective energy placement orpositioning or targeting 2301, 2303 of energy sources onto the donorand/or target wafers. For instance, an energy impluse(s) can be used toinitiate the cleaving action. The impulse (or impulses) is providedusing an energy source which include, among others, a mechanical source,a chemical source, a thermal sink or source, and an electrical source.

The controlled cleaving action is initiated by way of any of thepreviously noted techniques and others and is illustrated by way of FIG.11. For instance, a process for initiating the controlled cleavingaction uses a step of providing energy 2301, 2303 to a selected regionof the substrate to initiate a controlled cleaving action at theselected depth (z₀) in the substrate, whereupon the cleaving action ismade using a propagating cleave front to free a portion of the substratematerial to be removed from the substrate. In a specific embodiment, themethod uses a single impulse to begin the cleaving action, as previouslynoted. Alternatively, the method uses an initiation impulse, which isfollowed by another impulse or successive impulses to selected regionsof the substrate. Alternatively, the method provides an impulse toinitiate a cleaving action which is sustained by a scanned energy alongthe substrate. Alternatively, energy can be scanned across selectedregions of the substrate to initiate and/or sustain the controlledcleaving action.

Optionally, an energy or stress of the substrate material is increasedtoward an energy level necessary to initiate the cleaving action, butnot enough to initiate the cleaving action before directing an impulseor multiple successive impulses to the substrate according to thepresent invention. The global energy state of the substrate can beraised or lowered using a variety of sources such as chemical,mechanical, thermal (sink or source), or electrical, alone or incombination. The chemical source can include particles, fluids, gases,or liquids. These sources can also include chemical reaction to increasestress in the material region. The chemical source is introduced asflood, time-varying, spatially varying, or continuous. In otherembodiments, a mechanical source is derived from rotational,translational, compressional, expansional, or ultrasonic energies. Themechanical source can be introduced as flood, time-varying, spatiallyvarying, or continuous. In further embodiments, the electrical source isselected from an applied voltage or an applied electromagnetic field,which is introduced as flood, time-varying, spatially varying, orcontinuous. In still further embodiments, the thermal source or sink isselected from radiation, convection, or conduction. This thermal sourcecan be selected from, among others, a photon beam, a fluid jet, a liquidjet, a gas jet, an electro/magnetic field, an electron beam, athermoelectric heating, and a furnace. The thermal sink can be selectedfrom a fluid jet, a liquid jet, a gas jet, a cryogenic fluid, asuper-cooled liquid, a thermoelectric cooling means, an electro/magneticfield, and others. Similar to the previous embodiments, the thermalsource is applied as flood, time-varying, spatially varying, orcontinuous. Still further, any of the above embodiments can be combinedor even separated, depending upon the application. Of course, the typeof source used depends upon the application. As noted, the global sourceincreases a level of energy or stress in the material region withoutinitiating a cleaving action in the material region before providingenergy to initiate the controlled cleaving action.

In a preferred embodiment, the method maintains a temperature which isbelow a temperature of introducing the particles into the substrate. Insome embodiments, the substrate temperature is maintained between -200and 450° C. during the step of introducing energy to initiatepropagation of the cleaving action. Substrate temperature can also bemaintained at a temperature below 400° C. or below 350° C. In preferredembodiments, the method uses a thermal sink to initiate and maintain thecleaving action, which occurs at conditions significantly below roomtemperature.

A final bonding step occurs between the target wafer and thin film ofmaterial region according to some embodiments, as illustrated by FIG.12. In one embodiment, one silicon wafer has an overlying layer ofsilicon dioxide, which is thermally grown overlying the face beforecleaning the thin film of material. The silicon dioxide can also beformed using a variety of other techniques, e.g., chemical vapordeposition. The silicon dioxide between the wafer surfaces fusestogether thermally in this process.

In some embodiments, the oxidized silicon surface from either the targetwafer or the thin film of material region (from the donor wafer) arefurther pressed together and are subjected to an oxidizing ambient 2401.The oxidizing ambient can be in a diffusion furnace for steam oxidation,hydrogen oxidation, or the like. A combination of the pressure and theoxidizing ambient fuses the two silicon wafers together at the oxidesurface or interface 2305. These embodiments often require hightemperatures (e.g., 700° C.).

Alternatively, the two silicon surfaces are further pressed together andsubjected to an applied voltage between the two wafers. The appliedvoltage raises temperature of the wafers to induce a bonding between thewafers. This technique limits the amount of crystal defects introducedinto the silicon wafers during the bonding process, since substantiallyno mechanical force is needed to initiate the bonding action between thewafers. Of course, the technique used depends upon the application.

After bonding the wafers, silicon-on-insulator has a target substratewith an overlying film of silicon material and a sandwiched oxide layerbetween the target substrate and the silicon film, as also illustratedin FIG. 12 The detached surface of the film of silicon material is oftenrough 2404 and needs finishing. Finishing occurs using a combination ofgrinding and/or polishing techniques. In some embodiments, the detachedsurface undergoes a step of grinding using, for examples, techniquessuch as rotating an abrasive material overlying the detached surface toremove any imperfections or surface roughness therefrom. A machine suchas a "back grinder" made by a company called Disco may provide thistechnique.

Alternatively, chemical mechanical polishing or planarization ("CMP")techniques finish the detached surface of the film, as illustrated byFIG. 13. In CMP, a slurry mixture is applied directly to a polishingsurface 2501 which is attached to a rotating platen 2503. This slurrymixture can be transferred to the polishing surface by way of anorifice, which is coupled to a slurry source. The slurry is often asolution containing an abrasive and an oxidizer, e.g., H₂ O₂, KIO₃,ferric nitrate. The abrasive is often a borosilicate glass, titaniumdioxide, titanium nitride, aluminum oxide, aluminum trioxide, ironnitrate, cerium oxide, silicon dioxide (colloidal silica), siliconnitride, silicon carbide, graphite, diamond, and any mixtures thereof.This abrasive is mixed in a solution of deionized water and oxidizer orthe like. Preferably, the solution is acidic.

This acid solution generally interacts with the silicon material fromthe wafer during the polishing process. The polishing process preferablyuses a polyurethane polishing pad. An example of this polishing pad isone made by Rodel and sold under the tradename of IC-1000. The polishingpad is rotated at a selected speed. A carrier head which picks up thetarget wafer having the film applies a selected amount of pressure onthe backside of the target wafer such that a selected force is appliedto the film. The polishing process removes about a selected amount offilm material, which provides a relatively smooth film surface 2601 forsubsequent processing, as illustrated by FIG. 14.

In certain embodiments, a thin film of oxide 2406 overlies the film ofmaterial overlying the target wafer, as illustrated in FIG. 12. Theoxide layer forms during the thermal annealing step, which is describedabove for permanently bonding the film of material to the target wafer.In these embodiments, the finishing process is selectively adjusted tofirst remove oxide and the film is subsequently polished to complete theprocess. Of course, the sequence of steps depends upon the particularapplication.

In a specific embodiment, the silicon-on-insulator substrate undergoes aseries of process steps for formation of integrated circuits thereon.These processing steps are described in S. Wolf, Silicon Processing forthe VLSI Era (Volume 2), Lattice Press (1990), which is herebyincorporated by reference for all purposes. A portion of a completedwafer 2700 including integrated circuit devices is illustrated by FIG.15. As shown, the portion of the wafer 2700 includes active devicesregions 2701 and isolation regions 2703. The active devices are fieldeffect transistors each having a source/drain region 2705 and a gateelectrode 2707. A dielectric isolation layer 2709 is defmed overlyingthe active devices to isolate the active devices from any overlyinglayers.

Additional films may be separated from the donor substrate. For someapplications, the surface of the donor substrate does not needpreparation after a thin film has been cleaved off and before subsequentimplantation and cleaving steps occur. In other applications, it isbeneficial to prepare the surface of the donor substrate prior torepeating the cleaving sequence.

FIGS. 16 to 19 illustrate using a single donor substrate to producemultiple thin films. FIG. 16 shows the donor substrate 2105 after a thinfilm of material has been removed, as described above. An implant ofsecond particles 2110 has a desired concentration at a second selecteddepth z₁ to form a second material region 2112 to be removed. The secondmaterial region may be removed as a thin film following a process asdescribed above, such as a controlled cleavage process, a blisterseparation process, as well as others.

FIG. 17 shows the donor substrate 2105 being polished to improve thesurface finish of a cleaved surface 2116 prior to cleaving a second thinfilm of material from the donor substrate. The polishing operation issimilar to that described above for polishing a surface of a separatedfilm, and is generally done before the second implanting step.

FIG. 18 shows the donor substrate 2105 after a planarizing layer 2118has been applied to the cleaved surface 2116. The planarizing layer maybe a layer of plasma-etched deposited oxide, for example, spin-on glass,polyimide, or similar material. Preparing the surface of the donorsubstrate with a planarized layer of oxide or polymer prior to cleavinga subsequent thin film is desirable in some applications, especiallywhen using a transfer wafer or backing substrate. It is not necessary topolish the donor wafer prior to implantation, and the planarized surfaceof the deposited oxide or other materials provides a surface for bondingthe donor wafer to a transfer wafer by planarizing the minor surfaceimperfections of the cleaved surface of the donor wafer. Planarizing thedonor wafer in this fashion allows a donor substrate to be re-usedwithin a clean room environment, rather than sending the donor substrateout to be polished after each thin film has been cleaved.

For example, one process according to the present invention forfabricating multiple thin films from a single donor wafer using a waferbonding technique is described below:

(1) Provide a donor wafer;

(2) Implant particles into the wafer to define a first layer between asurface of the wafer and the particles;

(3) Bond the surface of the donor wafer to a first transfer wafer;

(4) Cleave a first thin film from the donor substrate, where the firstthin film adheres to the second transfer wafer;

(5) Deposit a planarized layer of silicon oxide on a cleaved surface ofthe donor wafer. Optionally, a layer of thermal oxide may be grown priorto the deposition and/or the donor substrate may be thermally treated toimprove the crystalline quality of the silicon;

(6) Implant additional particles into the wafer to define a second layerbetween the surface of the oxide layer and the particles;

(7) Bond the surface of the donor wafer to a second transfer wafer; and

(8) Cleave a second thin film from the donor substrate, the second thinfilm adhering to the second transfer wafer.

FIG. 19 is a simplified cross section of a substrate 2105 that with asecond region of material 2112 to be separated from the substrate. Aplanarizing layer 2118 has been applied to the cleaved surface 2116 toprepare it for bonding to the target wafer 2202. The planarizing layerprovides a good surface for a wafer bonding process, as described above.

Although the above description is in terms of a silicon wafer, othersubstrates may also be used. For example, the substrate can be almostany monocrystalline, polycrystalline, or even amorphous type substrate.Additionally, the substrate can be made of III/V materials such asgallium arsenide, gallium nitride (GaN), and others. The multi-layeredsubstrate can also be used according to the present invention. Themulti-layered substrate includes a silicon-on-insulator substrate, avariety of sandwiched layers on a semiconductor substrate, and numerousother types of substrates. Additionally, the embodiments above weregenerally in terms of providing a pulse of energy to initiate acontrolled cleaving action. The pulse can be replaced by energy that isscanned across a selected region of the substrate to initiate thecontrolled cleaving action. Energy can also be scanned across selectedregions of the substrate to sustain or maintain the controlled cleavingaction. One of ordinary skill in the art would easily recognize avariety of alternatives, modifications, and variations, which can beused according to the present invention.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A semiconductor substrate device comprising:adonor wafer having a cleaved surface; and a layer of particles disposedwithin said donor wafer a selected distance from and substantiallyparallel to said cleaved surface to define a layer of donor wafermaterial between said layer of particles and said cleaved surface. 2.The device of claim 1 wherein the cleaved surface is polished.
 3. Thedevice of claim 1 further comprising a dielectric layer disposed on saidcleaved surface.
 4. The device of claim 3 wherein the dielectric layeris planarized.
 5. The device of claim 3 wherein the dielectric layer isspin-on glass.
 6. The device of claim 3 wherein the dielectric layer isplasma-deposited silicon oxide.
 7. The device of claim 3 wherein thedonor wafer is a single-crystal silicon wafer.
 8. The device of claim 1wherein the selected distance is less than fifteen microns.
 9. Asemiconductor substrate device comprising:a single-crystal silicon donorwafer having a cleaved surface approximately parallel to a majorcrystallographic plane; and a layer of particles implanted into saiddonor wafer through said cleaved surface a selected distance from saidcleaved surface to define a layer of donor wafer material between saidlayer of particles and said cleaved surface.
 10. The device of claim 9wherein said particles are hydrogen ions.
 11. The device of claim 10wherein said hydrogen ions are implanted using plasma-immersion ionimplantation.
 12. The device of claim 9 wherein the majorcrystallographic plane is a {100} plane.
 13. The device of claim 9wherein the major crystallographic plane is a {110} plane.
 14. Thedevice of claim 9 wherein the major crystallographic plane is a {111}plane.
 15. The device of claim 9 wherein an as-grown defect density ofthe cleaved surface has been reduced to an annealed defect density by athermal treatment of the donor wafer.